1. Field of the Invention
The present invention relates to a two-step CMP (chemical-mechanical polishing) method and the employed polishing compositions and, more particularly, to a two-step CMP method and the employed polishing compositions suitable for planarizing wafer surfaces containing Al-alloy interconnections.
2. Description of Related Art
To satisfy a demand for high integration and multi-functions chips, the processes to form multi-layer metal interconnect thereon are developing. However, owing to the sever DOF limit for deep sub-micron photo-lithography, the wafer surfaces with interconnect topography need to be planarized via a polishing process, such as chemical-mechanical polishing (CMP).
In a CMP process, the removal rates, removal non-uniformity and surface roughness are mainly determined by the polishing compositions. For example, a proper formulation could be obtained by tuning solution pH, oxidants, and concentrations thereof, or various mechanical stress can be controlled by using different abrasives, varied particle sizes, and suspension conditions.
In general, the polishing conditions and slurries would be optimized in order to obtain higher throughput, and completely remove the barrier layer, such as Ti and TiN, outside of the wiring trenches. For example for polishing the Al-alloy layer, over 4000 xc3x85/min removal rates and less than 5% removal non-uniformity are required for the throughput taken into account. However, overpolish to insure against complete removal of Ti barrier would lead to sever Al dishing inside the wiring trenches and dielectric erosion around. Furthermore, dishing and erosion would be complicated with the pattern layout, especially for larger and denser patterns.
Therefore, it is desirable to provide an improved method to mitigate and/or obviate the aforementioned problems.
The main object of the present invention is to provide a two-step CMP method, which can rapidly planarize wafer surfaces.
The second object of the present invention is to provide CMP compositions, which can be employed in the above two-step method.
The further object of the present invention is to provide CMP compositions, which can be utilized for selectively polishing an Al-alloy layer or a barrier layer, so that metal dishing and erosion of dielectric layers can be avoided.
To achieve the above objects, the two-step CMP method of the present invention is performed first by polishing the Al-alloy layer in a first polishing slurry composed of one kind of abrasives, a buffer, a metal polishing promoter, an oxidant, and deionized water. Next, a second polishing slurry composed of one kind of abrasives, a pH buffer, a dielectric layer protector, an oxidant and deionized water is provided to polish the barrier layer.
For the first polishing slurry of the present invention, the abrasive, the oxidant, and the metal polishing promoter can be Al2O3, H2O2, and H3PO4 respectively. For the second polishing slurry of the present invention, the abrasive, the oxidant, and the dielectric layer protector can be Al2O3, H2O2, and tetra-butyl ammonium hydroxide (TBAH) respectively.
It is a further feature of the present invention that the barrier layer is selectively removed by inhibiting removal of the Al-alloy layer and the SiO2 dielectric layer in the second step. As a result, the problems such as metal dishing and erosion of dielectric layers often found in semiconductors with complicated pattern geometry are effectively solved.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description.
The present invention provides a CMP process carried out in two steps, in which two slurries are respectively employed to selectively remove the metal interconnection such as an Al-alloy layer, or the barrier layer such as a Ti/TiN layer. In such a process, a CMP slurry providing conventional functions is used in the first stage, plus an additional polishing slurry featured with the higher removal rates for polishing the barrier layer than for the metal layer is involved in the second stage.
Therefore, the chemical-mechanical polishing method of the present invention primarily includes steps of:
(A) polishing the metal layer with the first polishing slurry composed of one kind of abrasives, a pH buffer, a metal polishing promoter, an oxidant, and deionized water; and
(B) polishing the barrier layer with the second polishing slurry composed of one kind of abrasives, a buffer, a dielectric layer protector, an oxidant, and deionized water.
The abrasive aforementioned can be metal oxides, such as Al2O3, SiO2, CeO2, ZrO2, TiO2, and mixtures thereof.
The pH buffer used in the present invention can be oxalic, citric acid or salts thereof, for the purpose of control slurry pH to compensate pH variation from varied oxidative or corrosive environments, and to stabilize the removal rates of metals. For example, the removal rates of polishing Al-alloy at neutral slurry pH would be significantly dropped, but the removal rates of polishing Ti and TiN Would be enhanced with the alkaline slurry.
H3PO4 is functioned as a metal polishing promoter in the present invention.
The oxidant can be peroxides such as H2O2, transition metal complexes such as Fe(NO3)3 and K3Fe(CN)6, or halide peroxides such as ClO4xe2x88x92 and IO3xe2x88x92. Moreover, the removal rates of polishing metal are critically dependent upon the concentration of oxidants in the slurry. It is related to the corrosion and surface passivation of the metal being polished. For example, the removal rates of polishing Ti/TiN increase with the concentration of oxidants in slurry, however, the removal rates of polishing Al-alloy would be maximum only at a critical concentration of oxidants, but decreases in excess oxidants.
The dielectric layer protector of the present invention can be polar components such as glycol, or charged but non-polarized components such as tetrabutyl ammonium hydroxide (TBAH), tetramethyl ammonium, tetraethyl ammonium, ammoniates, and salts or hydroxides thereof.
The following Examples are used to explain the present invention, wherein Examples 1-4 refers to the removal selectivities of Al alloys, Ti, TiN, and silicon dioxide thin films. Examples 5-10 refers to the applications of polishing patterned wafers by the present invention.
The first polishing slurry used in the Examples is composed of 30-99 wt. % deionized water, 0.5-60 wt. % Al2O3 as the abrasive, 0.01-10 wt. % H3PO4 as the metal polishing promoter, and 0.1-30 wt. % H2O2 as the oxidant.
The second polishing slurry used in Examples is composed of 30-99 wt. % deionized water, 0.5-60 wt. % Al2O3 as the abrasive, 0.01-10 wt. % TBAH as dielectric layer protector, and 0.1-30 wt. % H2O2 as oxidant.
The end points of the polishing processes can be determined by polishing time, film thicknesses, electrical current changes of the carrier motor, or temperature changes on the polish pad.